Many performance parameters of batteries are associated with the electrolyte separator selected, and the interaction of the selected electrolyte with the cathode and anode materials used. All batteries require electrolytes to provide high ionic conductivity and electrochemical stability over a broad range of temperatures and potentials. Ionic conductivity is one of the most important properties of electrolytes. High ionic conductivity generally improves battery performance. Thus, significant research has focused on developing methods for increasing the ionic conductivity of electrolytes used in electrochemical cells.
The electrolyte used in lithium batteries can be a liquid or a polymer based electrolyte. Lithium batteries including liquid electrolytes have been on the market for several years. Lithium ion rechargeable batteries having liquid electrolytes are currently mass produced for applications such as notebook computers, camcorders and cellular telephones. However, lithium batteries having liquid electrolyte technology have several major drawbacks. These drawbacks relate to cost and safety and stem from use of a liquid electrolyte. The liquid electrolyte generally requires packaging in rigid hermetically sealed metal “cans” which can reduce energy density. In addition, for safety reasons, lithium ion rechargeable batteries and lithium-metal primary batteries having liquid electrolytes are designed to vent automatically when certain abuse conditions exist, such as a substantial increase in internal pressure which can be caused by internal or external overheating. If the cell is not vented under extreme pressure, it can explode because the liquid electrolyte used in liquid Li cells is extremely flammable.
Lithium batteries having solid polymer electrolytes represent an evolving alternative to lithium batteries having liquid electrolytes. Solid polymer electrodes are generally gel type electrolytes which trap solvent and salt in pores of the polymer to provide a medium for ionic conduction. Typical polymer electrolytes comprise polyethylene oxide (PEO), polyether based polymers and other polymers which are configured as gels, such as polyacrylonitrile (PAN), polymethylmethacrylate (PMMA) and polyvinylidine fluoride (PVDF). The polymer electrolyte generally functions as a separator, being interposed between the cathode and anode films of the battery.
Because its electrolyte is generally a non-volatile material which does not generally under normal operating conditions leak, a lithium battery having a polymer electrolyte is intrinsically safer than a lithium battery having a liquid electrolyte. Moreover, polymer electrolytes eliminate the need for venting and package pressure control which are generally required for operation of lithium batteries having liquid electrolytes. Thus, polymer electrolytes make it possible to use a soft outer case such as a metal plastic laminate bag, resulting in improvement in weight and thickness, when compared to liquid electrolyte can-type Li batteries.
Many performance parameters of lithium batteries are associated with the electrolyte choice, and the interaction of the selected electrolyte with the cathode and anode materials used. High electrolyte ionic conductivity generally results in improved battery performance. The ionic conductivity of gel polymer electrolytes have been reported to be as high as approximately 10−4 S/cm at 25° C. However, it is desirable for the ionic conductivity of the polymer electrolyte to reach even higher values for some battery applications. In addition, it would also be desirable to enhance the electrochemical stability of the polymer electrolyte towards anode and cathode materials to improve battery reliability, as well as storage and cycling characteristics.
While gel polymer electrolytes represent an improvement over liquid electrolytes in terms of safety and manufacturability, safety issues remain because gel polymers trap solvent on its pores and under extreme conditions (e.g. heat and/or pressure) can still escape and cause injury. In addition, gel polymer electrolytes cannot generally operate over a broad temperature range because the gel generally freezes at low temperatures and reacts with other battery components or melts at elevated temperatures. Moreover, electrode instability and resulting poor cycling characteristics, particularly for metallic lithium containing anodes, limits possible applications for such batteries formed with gel polymer electrolytes.
Alternative polymer materials have been actively investigated to provide improved characteristics over available polymer choices. For example, U.S. Pat. No. 5,888,672 to Gustafson et al. ('672 patent) discloses a polyimide electrolyte and a battery formed from the same which operates at room temperature and over a broad range of temperatures. The polyimides disclosed are soluble in several solvents and are substantially amorphous. When mixed with a lithium salt, the resulting polyimide based electrolytes provide surprisingly high ionic conductivity. The electrolytes disclosed in '672 are all optically opaque which evidences some phase separation of the various components comprising the electrolyte. Although the electrolytes disclosed by the '672 patent can be used to form a polymer electrolyte which provides an improved operating temperature range, ease of manufacture, and improved safety over conventional gel polymer electrolytes, it would be helpful if the electrolyte stability and ionic conductivity could be improved.